Study of Faradaic Reactions Occurring At Polycrystalline Platinum Electrode Surfaces in Frozen, Aqueous Acidic Electrolyte

Abstract

As the global demand for fuel cells in transportation vehicles continues to increase, it is important to understand how fuel cells operate under various environmental conditions. Currently little is known about interfacial properties and electrode behavior in frozen electrolytes. Such knowledge may help us understand how challenges associated with fuel cell operation in sub-­zero temperatures can be mediated. In the first part of this work, we report new results on the comparison of different Tafel plot acquisition methods for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and oxygen reduction reaction (ORR) at polycrystalline Pt (Pt(poly)) in frozen and liquid aqueous H2SO4 electrolyte. The compared methods are: (i) linear sweep, in which a slowly varying overpotential (η) is applied, (ii) steady-­state (SS) in which η is applied for t = 60 s to establish a SS current density before a new η is applied and (iii) SS with an electrode conditioning step between each applied η. Comparing these commonly used methods provides insight into their relative accuracy and will be useful for electrochemists when deciding on an appropriate and reproducible experimental method. In the second part of this work, we study the mechanism and kinetics of the HER, HOR, OER and ORR occurring at Pt(poly) in frozen aqueous H2SO4 electrolyte by conducting SS Tafel analyses. An interesting observation is made in the case of the HER, namely current maxima that result in a Tafel slope sign change. In the third part of this work, we study the effects of frozen electrolyte on the electro-­adsorption and electro-­desorption of under-­potential deposited H (HUPD) as well as on the formation and reduction of surface oxide on Pt(poly) in frozen aqueous H2SO4 electrolyte by conducting cyclic voltammetry (CV) measurements. Thermodynamic state functions associated with the electro-­adsorption and electro-­desorption of HUPD are determined using the general electrochemical adsorption isotherm. Both the CV profiles and the state functions are affected by the electrolyte freezing. Overall, this work establishes a foundation for further research in frozen electrolyte electrochemistry, a new research direction hardly ever explored but very important for fuel cell technologies.